Take butter and flour in large pot and turn the heat on medium. Stir constantly with a whisk until flour gives its raw smell and turn the heat off. Remove the pot from the heat. Wait for five minutes and then add 1 cup of milk. Stir with a whisk until flour is not lumpy. Add the other 3 cups of milk, vanilla, and sugar and turn the heat on high. Stir constantly until smooth consistency and turn the heat off.In a rectangular pyrex, layer graham cookies side by side and spoon pudding over. Repeat this step three times.When it is finished, there will be 3 layer cookies and 3 layer pudding. Set a side until it is completly cool.Take chocolate chips and milk in a small skillet and turn the heat on medium high. Stir until it is smooth. Spoon the sauce over the cookies-pudding layers. Chill in the refrigerator before serving.